Beamspread measurement camera for recording laser beam intensity distribution

ABSTRACT

Energy distribution within a laser beam is measured and a sample of the intensity is obtained, providing a self-calibrated photograph on a single film sample. A burst of laser energy striking a wedge plate or prism within the beamspread measurement camera is divided into multiple reflections within the prism. Each reflection is at a slightly greater angle than the preceeding one and is attenuated with respect thereto. Each reflection is collected by a lens and focused onto a spectrographic plate located at the lens focal point. The diameter of each image on the film is the product of the lens focal length and beamspread. The distribution pattern of each reflected sample is recorded on the same plate and can be compared with preceeding samples, each sample being a known percentage of the preceeding waves. Relative energy as a function of beamspread is determined by measuring the energy density of each image with a densitometer. Any point on each image of reflected energy is a known percentage of that point on the other images and provides a standard for determining the contour ring for a particular energy density on the initial spot, thus providing a contour map of the main beam.

United States Patent Dilworth et a].

[54] BEAMSPREAD MEASUREMENT CAMERA FOR RECORDING LASER BEAM INTENSITYDISTRIBUTION [72] Inventors: James B. Dilworth, Birmingham, Ala.; ThomasG. Crow, Orlando, Fla.

[73] Assignee: The United States of America as represented by theSecretary of th Army [22] Filed: Dec. 10, 1970 [21] Appl. No.: 96,807

[52] US. Cl. ..356/235, 356/202, 356/213, 356/256 [51] Int. Cl. ..G0lj1/40, GOln 21/06, GOlj 1/00 [58] Field of Search...350/160 R; 356/215,201, 202, 356/203, 213, 234, 235, 256

Primary Examiner-Ronald L. Wibert Assistant Examiner-V. P. McGrawAttorney-Harry M. Saragovitz, Edward J. Kelly and Herbert Bell [451 Aug.1, 1972 [57] ABSTRACT Energy distribution within a laser beam ismeasured and a sample of the intensity is obtained, providing aself-calibrated photograph on a single film sample. A burst of laserenergy striking a wedge plate or prism within the beamspread measurementcamera is divided into multiple reflections within the prism. Eachreflection is at a slightly greater angle than the preceeding one and isattenuated with respect thereto. Each reflection is collected by a lensand focused onto a spectrographic plate located at the lens focal point.The diameter of each image on the film is the product of the lens focallength and beamspread. The distribution pattern of each reflected sampleis recorded on the same plate and can be compared with preceedingsamples, each sample being a known percentage of the preceeding waves.Relative energy as a function of beamspread is determined by measuringthe energy density of each image with a densitometer. Any point on eachimage of reflected energy is a known percentage of that point on theother images and provides a standard for determining the contour ringfor a particular energy density on the initial spot, thus providing acontour map of the main beam.

CALORIMETER PATENTED AUG 1 I972 CALORIMETER li l FIG. I

R mox= l/2Imox I/4I max l/8I max l/IGI max FIG. 3

James B. Dilworth Thomas G. Crow INVENTO BY 7 BEAMSPREAD MEASUREMENTCAMERA FOR RECORDING LASER BEAM INTENSITY knowledge of laser beamintensity and the amount of beamspread or divergence are important indetermining consistance of energy distribution and power output of thelaser. When the laser beamspread is known the amount of energy requiredto provide a prescribed signal strength at a given distance from thelaser can be determined. A narrow beamspread provides high energydensity. In the past a block of wood, photo-film or other substance hasbeen shot with a burst of energy to provide some indication of the spotsize and energy distribution by examining the charred or discoloredspot. This indicated only where the maximum energy density or hot spotwas located. The energy distribution in a beam has also been measured byblocking most of the beam and allowing a portion to pass through anaperture and into a calorimeter. This is repeated for various bursts ofenergy with the aperture moved to various locations in the beam or withthe aperture size changed. This requires several bursts of energy andprovides only spots of energy density in the initial image, the locationof which may have changed in succeeding bursts of energy. Also the totalenergy content of successive bursts will not be precisely the same.

Beam divergence measurements are currently made at various lasingWavelengths by recording the beam image on commercially availablespectrographic plates. Several images are required and beam intensitymust be reduced to prevent film damage if a high energy density beam ismeasured. Distribution of the energy in a beam indicates the quality ofthe optical system of the laser. Replacing or relocating an optical pumpor lens, for example, may change the energy distribution within a laserbeam. Similarly, the gradual decay of an optical component causeschanges in the beam density. Thus, the distribution of energy within abeam indicates the uniformity or non-uniformity of the output beam.Because of small changes that occur in the energy pattern withsucceeding shots during the lifetime of a lasers optical components,measurement and calibration of laser beam energy cannot be accuratelyrecorded from a series of pulses or bursts of energy.

SUMMARY OF THE INVENTION In a beamspread measurement camera, the energydistribution within a pulse of laser energy is sampled to provide aself-calibrated photographic image on a single film sample. A prismwithin the camera is partially reflective on one side, allowing a burstof laser energy impinging thereon to be partially reflected toward arecording film and partially refracted therein. The refracted wavewithin the prism undergoes multiple reflections therein with apercentage of each reflected wave being transmitted out of the prism andrecorded on the film adjacent the initially reflected portion of thewave. Each succeeding sample of energy can be a fixed percentage ofpreceding waves. By determining the density of the initial image andsucceeding images, the distribution of energy density around the centerof the beam can be determined. The maximum point or points of densityand density rings of equal energy levels around the maximum points aredetermined from this single pulse of laser energy, providing acalibrated beamspread from a single laser pulse. Relative energy as afunction of beamspread can be calculated for all classes of lasers,since the diameter of each image on the film is the product of a lensfocal length and beamspread and since each image is attenuated by aknown amount.

An object of this invention is to provide a beamspread measurementcamera for recording a plurality of energy density images from a singlelaser pulse.

Another object of the present invention is to provide a camera forrecording succeeding reflections of laser energy for providing agradient density contour across the laser beam image.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a preferred embodiment of abeamspread measurement camera showing the optical path therein.

FIG. 2 is a view of a partially reflecting prism for reflecting laserenergy.

FIG. 3 is a contour map of a laser pulse intensity divergence.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,like numerals represent like parts in each figure. In FIG. 1 abeamspread measurement camera 10 receives a burst of laser energy 12.The laser pulse passes through a filter opening 14 in the camera andimpinges on a prism 20. Part of incident wave 12 is reflected by prism20 and directed toward a lens 22. The remainder of wave 12 undergoesmultiple reflections within prism 20 with a portion of each reflectedwave being coupled out of the prism and directed toward lens 22 duringalternate reflections. Each successive beam passing through lens 22 isdirected toward a mirror 24, which directs the laser energy toward aspectrographic plate or film 26 located at the focal point of the lens.By using mirror 24 to fold the laser beam, the camera housing isconstructed without the excessive length that would otherwise be neededto focus the beam on the film. To ensure that plate 26 receives only thereflected beam energy, a chamber wall 28 separates plate 26 from theprism and focusing lens. A calorimeter 30 can be used to measure thetotal energy level by placing a beamsplitter 32 in the path of theincident laser pulse and directing a sample of the energy toward thecalorimeter.

In FIG. 2, prism 20 is shown with an incident wave I and reflected wavesR1, R2, R3 and R4. The percentage of energy reflected during eachalternate wave is dependent on the energy that the spectrographic platecan record without deterioration of the plate, and the number of densitylevels desired for each burst of energy. The partially reflective frontsurface of prism 20 can be any required percentage of the incidentenergy. For example, with a percent reflective back surface and a 50percent reflective front surface, alternate reflective waves have 50percent of the energy intensity of the preceding wave striking thepartially reflective surface. Thus, wave R1 equals one-half I and thefirst internal reflection is one-half I. Wave and the third internalreflection is one-fourth l. A selection of replacable prisms can providea choice of the percentage reflection to be used with a given laser.

The reflecting surfaces of prism 20 may be displaced from the parallelby a very minute angle 0, such as or 0 30. The amount of angulardisplacement of these reflecting surfaces determines the degree ofseparation of each image on plate 26 for a given angle of incidence ofbeam 1. After an incident wave 12 strikes the partially reflectivesurface of prism 20, each reflection within the prism is at a slightlygreater angle than the preceding reflection and is attenuated by thecoupling out of energy to the lens. In practice the angle 0 is usuallyless than a degree. However, for purposes of illustration in FIG. 2,wedge has front and rear surfaces forming an angle 0 of approximately 4.Incident wave 12 strikes the front surface of wedge 20 at an acute angle(1), with respect to a normal to the surface. The reflected energy Rlfrom wave 12, having the same angle of reflection as the angle ofincidence, reflects away from the wedge at the angle 4),. The refractedbeam portion 12a (one-half l) strikes the wedge rear surface at point 40and is totally reflected back toward the front surface. Prior toreflection, wave 12a strikes the rear surface at an angle with respectto the surface normal, 45 2 6. After being reflected, wave 12a ispartially refracted at the wedge front surface to provide beam R2, theremainder thereof being internally reflected as beam 12b at the angle 0.This action continuesfor each internal reflection. Since two additionalreflections have occurred when the beam reaches point 42, the angle ofincidence (p (1) 20. Similarly, (p (p 20 at point 44. Assuming forexample that beam 12a strikes point 40 at an angle of 10 from thenormal, 10, (1);; 18, and 4:, 26. Therefore, the internally reflectedbeams are partially transmitted at different angles with respect to therear surface normal or other common reference. With respect to the rearsurface of wedge 20, the approximate angles of reflection from thenormal are 10 for R2, 18 for R3 and 26 for R4. Since each reflected waveR is displaced in space with respect to adjacent waves it is focused insucceeding stages on plate 26, forming respective images thereon. Afterseveral reflections the energy remaining within the prism is negligible.This remaining energy escapes at the end of the prism and is absorbed bythe black inner surfaces of the camera chamber.

The beamspread of a laser beam is determined by the angle at which theenergy density decreases to one-half of the maximum density. For goodhomogeneous pumping and lasing action this maximum intensity point isthe center of a circle, and a rapidly decreasing gradient energy aroundthe circle decreases to zero. Distribution of energy within a beamindicates the quality of optical components, such as the pumping means,lens and mirrors. Gradual changes in the optical system result inchanges in the beamspread. For small changes in laser beam divergence,there is a nonlinear loss in energy density which should be compensatedfor. Periodic single pulses of energy recorded on film 26 provide acurrently accurate beamspread measurement that is indicative of thelaser beam energy dis tribution.

Relative energy of the laser beam as a function of beamspread isdetermined from the known camera parameters. The diameter of each imageon the film is the product of the lens focal length and beamspread. Eachimage is attenuated by the known percentage of reflectivity of the prismfront surface. Densitometry of the images on the spectrographic platesallows construction of an energy density contour map. A densitometerscan of each image reveals the energy density distribution therein. Forthe 50 percent reflective prism surface the maximum energy in the secondreflection image R2 is 50 percent of the maximum energy of the firstreflection Rl. Similarly, the maximum energy in the image of R3 isone'fourth R1. Therefore, a curve at the respective percentage of energylevels can be drawn around the maximum point of the initial imageallowing quantitative measurement of the relative energy densitydistribution of a laser beam from a single pulse of the laser. The filmor plate 26 is calibrated against itself regardless of the type ofplates recorded on, eliminating any need to refer back to a calibrationstandard. FIG. 3 is a contour map that is typical of the first recordedimage from a partially reflected laser pulse. Contour rings for eachenergy intensity level recorded is a percentage of the initial wave 1.The calorimeter measurement of total energy provides a means ofcomparison with the image of energy intensity but does not provide thespectrum of energy density across the beam as provided by the beamspreadmeasurement camera.

Although a particular embodiment and form of this invention has beenillustrated, it is obvious to those skilled in the art thatmodifications may be made without departing from the scope and spirit ofthe foregoing disclosure. Therefore, it is understood that the inventionis limited only by the claims appended hereto.

We claim:

1. Beamspread measurement apparatus for recording energy density of alaser beam comprising: a camera housing having an opening therein fordirecting a coaxially aligned incident laser beam therethrough,reflecting means within a first chamber of said housing and coaxiallyaligned with said opening for reflecting consecutive percentages of saidlaser beam, recording means within a second chamber of said housing, anda focusing lens within said housing for receiving said percentages oflaser energy and directing said energy toward said recording means.

2. Beamspread measurement apparatus as set forth in claim 1 wherein saidreflecting means is a first prism having a reflective rear surface and apartially reflective front surface, said prism being positioned to allowmultiple reflections of said laser beam therein with each reflectionbeing at a slightly greater angle and attenuated with respect to apreceding reflection.

3. Beamspread measurement apparatus as set forth in claim 2 wherein saidrecording means is a spectrographic film at the focal point of said lensfor recording an image of each reflection focused thereon.

4. Beamspread measurement apparatus as set forth in claim 3 and furthercomprising a mirror between said lens and said film for folding saidlaser beams into said second chamber, a calorimeter for measuring thepower of said laser beam and a beam splitter adjacent said cameraopening for directing a portion of said laser energy to saidcalorimeter.

5. Beamspread measurement apparatus as set forth energy during eachalternate reflection, in claim 4 wherein said reflecting means furthercomc. deflecting a percentage of said incident wave and a Pnses a Pl ofP P each P P bemg adapted percentage of each alternate reflectionthrough a for replacing said first prism and having the separate lens,and distinct combination of a partially reflective front 5 focusing Saidd fl t d energy beams on a spectresurface and an angle of deflection forvarying the pergraphic plate and recording Separate images for E 9posmon of Fespecuve laser beam reflec each deflected beam of theoriginal incident beam, Hons to Sald Spectrograph]? fi e. determiningthe maximum density of each A method for de.termmmg the relauve enqgy ofa recorded image and the gradient density of said inlaser beam as afunction of beamspread comprising the 10 cidem deflected wave image andsteps of: f

t aid a. directing a burst of mcldent laser energy toward a f1322;232:21 2: zgf ilgzg gg prism,

b. reflecting a decreasing percentage of said energy within said prismat successively increasing angles and simultaneously attenuating saidreflected maximum energy for each succeeding reflected wave image.

1. Beamspread measurement apparatus for recording energy density of alaser beam comprising: a camera housing having an opening therein fordirecting a coaxially aligned incident laser beam therethrough,reflecting means within a first chamber of said housing and coaxiallyaligned with said opening for reflecting consecutive percentages of saidlaser beam, recording means within a second chamber of said housing, anda focusing lens within said housing for receiving said percentages oflaser energy and directing said energy toward said recording means. 2.Beamspread measurement apparatus as set forth in claim 1 wherein saidreflecting means is a first prism having a reflective rear surface and apartially reflective front surface, said prism being positioned to allowmultiple reflections of said laser beam therein with each reflectionbeing at a slightly greater angle and attenuated with respect to apreceding reflection.
 3. Beamspread measurement apparatus as set forthin claim 2 wherein said recording means is a spectrographic film at thefocal point of said lens for recording an image of each reflectionfocused thereon.
 4. Beamspread measurement apparatus as set forth inclaim 3 and further comprising a mirror between said lens and said filmfor folding said laser beams into said second chamber, a calorimeter formeasuring the power of said laser beam and a beam splitter adjacent saidcamera opening for directing a portion of said laser energy to saidcalorimeter.
 5. Beamspread measurement apparatus as set forth in claim 4wherein said reflecting means further comprises a plurality of prisms,each prism being adapted for replacing said first prism and having theseparate and distinct combination of a partially reflective frontsurface and an angle of deflection for varying the percentage andposition of respective laser beam reflections to said spectrographicfilm.
 6. A method for determining the relative energy of a laser beam asa function of beamspread comprising the steps of: a. directing a burstof incident laser energy toward a priSm, b. reflecting a decreasingpercentage of said energy within said prism at successively increasingangles and simultaneously attenuating said reflected energy during eachalternate reflection, c. deflecting a percentage of said incident waveand a percentage of each alternate reflection through a lens, d.focusing said deflected energy beams on a spectrographic plate andrecording separate images for each deflected beam of the originalincident beam, e. determining the maximum density of each recorded imageand the gradient density of said incident deflected wave image, and f.indicating a contour ring of energy density on said incident deflectedimage corresponding to the maximum energy for each succeeding reflectedwave image.